Question: Solve for $x$ and $y$ using substitution. ${-2x-y = -12}$ ${y = x-9}$
Since $y$ has already been solved for, substitute $x-9$ for $y$ in the first equation. ${-2x - }{(x-9)}{= -12}$ Simplify and solve for $x$ $-2x-x + 9 = -12$ $-3x+9 = -12$ $-3x+9{-9} = -12{-9}$ $-3x = -21$ $\dfrac{-3x}{{-3}} = \dfrac{-21}{{-3}}$ ${x = 7}$ Now that you know ${x = 7}$ , plug it back into $\thinspace {y = x-9}\thinspace$ to find $y$ ${y = }{(7)}{ - 9}$ $y = -2$ You can also plug ${x = 7}$ into $\thinspace {-2x-y = -12}\thinspace$ and get the same answer for $y$ : ${-2}{(7)}{ - y = -12}$ ${y = -2}$